The present invention relates to a thermoplastic resin composition rich in heat resistance and oil resistance and excellent in tensile elongation properties.
Styrene resins are used widely as an injection molding material or sheet forming material because of its rigidity, good stability and low cost. They however have stress cracks under the influence of a fatty oil or organic solvent and undergo a drastic deterioration in physical properties. In addition, since they are amorphous and have a glass transition point of about 90xc2x0 C., they easily change in shape at a relatively low temperature not greater than 90xc2x0 C. These drawbacks prevent free use of them.
There has accordingly been an attempt to prepare a resin composition having improved oil resistance and chemical resistance by adding, to a styrene resin, a propylene resin having excellent oil resistance. Proposed in JP-A-54-53159 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) and JP-A-54-13354 are compositions obtained by adding a compatibilizer to a composition of a styrene resin and a propylene resin, which have inherently difficulty in mixing, in order to improve their compatibility. For example, in JP-A-54-53159, proposed is a thermoplastic composition obtained from a polystyrene resin and a polyethylene or poly-xcex1-olefin resin by using, as a compatibilizer, a selectively hydrogenated linear, sequential or radial teleblock copolymer which comprises a polymerized aromatic vinyl compound A and an olefin elastomer B and has a structure of A-B-A, A-(B-A-B)n-A, A(BA)nB, B(A)4 or B[(AB)nB]4 (wherein, n stands for an integer of 1 to 10). In the block copolymer serving as a compatibilizer, a block of the polymerized aromatic vinyl compound preferably accounts for 15 to 40 wt. % of all the block polymers, the olefin elastomer B is preferably a hydrogenated saturated polybutadiene, and 30 to 50% of butadiene carbon atoms are vinyl side chains. As its specific example, an SEBS block copolymer having a 30 wt. % of a bound styrene content is disclosed. However, owing to a low content of the aromatic vinyl compound block, this block copolymer lacks affinity with a styrene resin. An amount of the block copolymer which can exist on the interface as a compatibilizer for the styrene resin and propylene resin is not sufficient so that the resulting composition is poor in tensile elongation properties. In JP-A-56-38338, on the other hand, proposed is a thermoplastic resin composition formed of a polyolefin resin and a polystyrene resin by using, as a compatibilizer, a hydrogenated block copolymer available by hydrogenating a block copolymer having at least one aromatic vinyl compound polymer block A and at least one conjugated diene polymer block B and containing 15 to 85 wt. % of a bound vinyl-substituted aromatic compound, thereby saturating at least 70% of double bonds in the block copolymer. In Examples, disclosed as a compatibilizer for a composition of a styrene resin and a propylene resin is A-B-A-B tapered 4 type hydrogenated block copolymers having a bound styrene content of 40 wt. % and a vinyl content, before hydrogenation, of 13%. These hydrogenated block copolymers involve drawbacks such as complex structure and cumbersome preparation process. There is accordingly a demand for a hydrogenated block copolymer which has a simple structure and can be prepared easily. In JP-A-5-186660 and JP-A-6-184366, proposed are resin compositions having excellent heat resistance, chemical resistance and oil resistance and formed from a rubber-modified styrene resin and a polyolefin resin by using, as a compatibilizer, a hydrogenated triblock copolymer which comprises a styrene block, isoprene block, and a styrene block, with these styrene blocks accounting for 40 to 70 wt. % of these blocks. The hydrogenated triblock copolymer having an isoprene block is however inferior in weather resistance and heat resistance and has a problem in recycling properties. In addition, these resin compositions are not always satisfactory in tensile elongation properties, because adhesion of the styrene resin phase and propylene resin phase on the interface therebetween is insufficient.
An object of the invention is to provide a thermoplastic resin composition rich in heat resistance and oil resistance and having excellent tensile elongation properties by adding, to a styrene resin and a propylene resin, a hydrogenated block copolymer having a specific structure.
The object of the invention has been achieved by a thermoplastic resin composition comprising (A) a styrene resin, (B) a propylene resin and (C) a hydrogenated block copolymer comprising at least two polystyrene blocks Xs and at least one polybutadiene block Y, with at least 70% of the double bonds of the polybutadiene of the polybutadiene block Y having been hydrogenated, wherein a styrene content of the hydrogenated block copolymer (C) is 40 to 80 wt. %, a 1,2-bound content of the polybutadiene block is 30 to B80 wt. %, a weight ratio of the component (A) to the component (B) is 95:5 to 5:95, a content of the component (C) is 2 to 30 parts by weight based on 100 parts by weight of the components (A) and (B), and at least 50% of the component (C) exists on the interface between the phase of the component (A) and the by phase of the component (B).